The present invention relates to a method for verifying the plausibility of a measuring device for determining a quality of a urea aqueous solution in a reservoir of an SCR catalyst system. In addition, the present invention relates to a computer program which executes all steps of the inventive method if said method is run in a computing device or control device. Finally the present invention relates to a computer program product comprising a program code, which is stored on a machine-readable carrier, for carrying out the inventive method if the program is executed on a computer or in a control device.
In order to comply with increasingly stringent exhaust emission legislation, it is necessary to reduce nitrogen dioxides in the exhaust gas of internal combustion engines, in particular diesel engines. To this end, it is known to dispose an SCR catalyst (selective catalytic reduction) in the exhaust gas region of internal combustion engines, said catalyst reducing nitrogen oxides (NOx) contained in the exhaust gas of an internal combustion engine to nitrogen in the presence of a reducing agent. The proportion of nitrogen oxide in the exhaust gas can thereby be considerably reduced. Ammonia (NH3), which is added to the exhaust gas, is required for the course of the reaction. For that reason, NH3 or NH3-releasing reagents are used as the reducing agent. As a rule, a hydrous urea solution (aqueous urea solution) is used which is injected into the exhaust gas tract upstream of the SCR catalyst. Ammonia forms from this solution which acts as reducing agent. A 32.5% aqueous urea solution is commercially available under the brand name AdBlue®.
In the case of commercial vehicles, the aqueous urea solution is refilled by the driver. A refilling of the reagent by the driver, which occurs outside of specified maintenance intervals, is also being discussed for passenger vehicles in the future. A check as to whether the refilled substance actually conforms to aqueous urea solutions or, respectively, as to whether the urea concentration in the aqueous urea solution corresponds to the relevant ISO standard 22241 or to the DIN standard 70070 has not occurred to date. The lawmakers will, however, demand in the future that a check is immediately made to detect whether an attempt to defraud was made when refilling the aqueous urea solution and that corresponding measures, such as, for example, a restriction of engine start-up or a reduction in engine performance, can be introduced if fraud were detected. For this purpose, the systems will have to be expanded to include a quality sensor which determines the quality of the aqueous urea solution. An SCR system comprising such a quality sensor is known, for example, from the German patent application DE 101 39 142 A1. This document does not, however, describe how a malfunction of the quality sensor can be detected in order to prevent an erroneous detection of a deviating aqueous urea solution or a filling of the reservoir with a substance which does not have the properties of an aqueous urea solution.